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Effect of pH, DIC, Orthophosphate and Sulfate on Drinking Water Cuprosolvency
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Effect of pH, DIC, Orthophosphate and Sulfate on Drinking Water Cuprosolvency (PDF) (107 pp, 561KB)
ABSTRACT
Effect of pH, DIC, Orthophosphate and Sulfate
on Drinking Water Cuprosolvency
EPA/600/R-95/085
June 1995
Field data from various copper monitoring studies and Lead and Copper Rule compliance data are often inappropriate and misleading for reliably determining fundamental chemical relationships behind copper corrosion control. To address this deficiency, a comprehensive solubility model for copper in drinking water has been developed, that is qualitatively and quantitatively consistent with available data for copper dissolution and precipitation. The concentration of Cu(I) is dominated by Cu2O(s) or CuOH(s) solid phases, plus soluble aqueous ammonia and chloride complexes. For young piping, the concentration of Cu(II) is mainly governed by Cu(OH)2(s) (cupric hydroxide), rather than CuO(s) (tenorite) or Cu2(OH)2CO3(s)(malachite). Complexation of Cu(II) by DIC and hydroxide ion is extremely important. Increases in DIC are predicted to cause significant increases in copper solubility in the pH range of 7 to 10. Often, utilities must trade off increasing cuprosolvency by DIC addition for ensuring adequate buffering intensity in the finished water. Sufficient dosages of orthophosphate in the pH range of 6.5 to 7.5 may reduce cuprosolvency, while sulfate may assist in controlling cuprosolvency under some chemical conditions, or may interfere with the formation of cupric hydroxide films under mildly alkaline conditions. Dissolved oxygen and chlorine residual play complicated roles in determining copper concentrations after various standing times. Frequently, 48 to 72 hours are necessary to reach equilibrium levels of copper in disinfected systems. Considerable uncertainties in much of the important thermodynamic data for copper species still exist, and many research needs remain that are critical to improving the understanding and control of cuprosolvency. This report covers a period from June 1991 to November 1993, and work was completed as of May 1995.